The present invention generally relates to filters, and more particularly to an acoustic wave filter comprised of a Surface Acoustic Wave (SAW) resonator and a Bulk Acoustic Wave (BAW) resonator and a method of forming the same.
The communications industry has placed a great deal of emphasis upon miniaturizing communications devices such as portable phones, radios and the like. Simultaneously, the industry has increased the complexity of such devices. Therefore, filters within such devices are sought that exhibit properties such as smaller size, lighter weight, lower cost and higher performance. One example of such a filter is a radio frequency (RF) front-end filter for use in a duplexer of a small portable radio or phone.
Acoustic wave filters that are formed of all Surface Acoustic Wave (SAW) or all Bulk Acoustic Wave (BAW) resonators are typically small and lightweight and exhibit some high performance characteristics. (See U.S. Pat. No. 5,854,579, titled xe2x80x9cSaw Filter Using Low-Pass Configuration and Method of Providing the Same,xe2x80x9d issued to David Penunuri on Dec. 29, 1998, for an example of an all SAW filter, which is hereby incorporated by reference, and U.S. Pat. No. 5,872,493, titled xe2x80x9cBulk Acoustic Wave Filter Having a Top Portion that Includes a Protective Acoustic Mirror,xe2x80x9d issued to Juha Ella on Feb. 16, 1999, for an example of a BAW filter, which is hereby incorporated by reference, and hereinafter referred to as the xe2x80x9cElla Referencexe2x80x9d). For example, SAW filters are known to exhibit high coupling coefficients and, therefore, provide superior rejection of undesired frequencies. BAW filters, on the other hand, are known to maintain a low insertion loss and to have low temperature coefficients, which prevent drift out of the transition bands at elevated temperatures. (See U.S. Pat. No. 5,933,062, titled xe2x80x9cAcoustic Wave Ladder with Effectively Increased Coupling Coefficient and Method of Providing the Same,xe2x80x9d issued to Richard S. Kommrusch on Aug. 3, 1999, for additional discussion of filter properties, which is hereby incorporated by reference.)
While acoustic wave filters that are formed of all SAW or all BAW resonators are typically small, light weight, and exhibit some high performance characteristics, these all SAW or all BAW resonator filters suffer from performance weaknesses. For example, a SAW filter may suffer from high insertion loss or excessive drift out of the transition band due to high temperature coefficients. BAW filters, on the other hand, may suffer from low coupling coefficients, resulting in poor rejection of undesired frequencies.
Accordingly, it is desirable to provide an acoustic wave filter that combines SAW and BAW resonators to form a filter that takes advantage of the respective strengths of the two different resonators and minimizes the weaknesses. Furthermore, it is desirable to provide an efficient method for forming such a filter. Additionally, other desirable characteristics will become apparent to one skilled in the art from the foregoing background of the invention, the following detailed description of the drawings and the appended claims.